International patent application no. PCT/AU99/00713 in the name of the present applicants describes a laser-based analyser. This analyser can be used to analyse solid, liquid or even gaseous materials. However, the analyser is most suitably used for analysing solid materials, such as coal.
In the analyser described in international patent application no. PCT/AU99/00713, laser radiation is focussed onto a sample, which causes the sample to emit light. The emitted light is detected by a plurality of detectors. The plurality of detectors detects information from specific parts of the spectral region and forwards that information to respective data collecting means.
An embodiment of the analyser described in international patent application no PCT/AU99/00713 is described with reference to FIG. 1 attached hereto.
In the apparatus shown in FIG. 1, a laser 10, which may be a 1064 mm ND:YAG laser, emits pulses of laser light that are focussed by an optical system 12 onto a material to be analysed 14. In the small region of the laser spot focused on the material 14, the laser power density produces rapid heating and ionisation of a small sample of the material. Light is emitted from the vaporised and ionised material containing spectral information on the material involved. The light emitted from the vaporised and ionised material is schematically represented at 16 and this emitted light is detected by a plurality of detection means 20, 22, 24. The apparatus shown in FIG. 1 has three detection means but it will be appreciated that a lesser or greater number of detection means may be utilised. It is envisaged that a greater number of detection means may be utilised if especially high resolution is required. Detection means 20 comprises a spectrometer 26 that is adjusted to a part of the spectrum of the spectral emissions emanating from material 14. Detection means 20 also includes a CCD detector 21 which suitably comprises a readily available commercial CCD detector. The CCD detector 21 may comprise a 12–16-bit detector.
Similarly, detection means 22 comprises a spectrometer 30 and a CCD detector 32. Detection means 24 also comprises a spectrometer 34 and CCD detector 36.
The CCD detectors 21, 32, 36 detect information from the specific spectral region provided by their associated spectrometers. The CCD detectors then pass the detected information to respective dedicated data acquisition means 38, 40, 42. The data acquisition means may include analog-to-digital conversion boards/circuitry. The computer 44 also includes control means 46 to control the operation of the laser 10 and the plurality of the detection means 20, 22, 24.
In use of the apparatus shown in FIG. 1, the control means 46 sends a control signal to laser 10 which causes the laser to emit a pulse of laser light. The pulse of laser light 10 is focused onto the surface of material 14 which causes vaporisation and ionisation of a small part of the material 14.
Shortly after the control signal causes a pulse of laser light 10 to be emitted by the laser, the control means 46 sends control signals to the detection means 20, 22, 24 which turns on those detection means. It is preferred that there is a slight delay between firing of the laser and initialisation of operation of the spectrometers in order to ensure that the CCD detectors do not detect the pulse of laser light and only detect the emitted spectra. This control signal causes the spectrometers 26, 30, 34 to collect light from the relevant spectral region for a predetermined period of time and to enable the CCD detectors 21, 32, 36 to detect that light. Each of spectrometers 26, 30, 34 collect light from particular regions of the emission spectrum. The particular regions may be discrete, separate regions of the spectrum, or there may be some overlap between the spectral region collected by one of the spectrometers and the spectral region collected by another of the spectrometers. Whilst the detection means 20, 22, 24 are collecting and detecting the light from the emitted spectral region from the sample 14, the CCD detectors are also forwarding information to the respective data acquisition means 38, 40, 42. The CCD detectors are formed from individual areas of light sensitive material (usually silicon) known as pixels. Each pixel converts the light intensity to an electric change or current which is then digitised by the data acquisition means. The use of separate data acquisition means for each detection means enables rapid collection of large amounts of data and this in turn allows the rapid analysis of the material to take place at high spectral resolution.
The data collected by the data acquisition means 38, 40, 42 is then analysed by the computer to determine the elements of species present in the material and also to determine the relative amounts of each of those elements or species. The amount of each element or species in the material may be determined by integrating the area under the spectral line at a wavelength that is characteristic of the spectral emission of a given element or species and comparing that area with the area under the same spectral line obtained from a material having a known content of that particular element or species.